Method of and apparatus for testing the hearing



Mmh 1o, 192s. 1,528,774

F. W. KRANZ METHOD OF AND APPARATUS FOR TESTING THE` HEARING' Filed Nov. 20, 1922 Patented Mar. 10, v1925.

, UNITED STATES PATENT oEFl'cE.

FREDERICK W. KRANZ, OF GENEVA, ILLINOIS.

METHOD OF AND APPARATUS FORTESTINGYTEE HEARING.

Appation led November 20, 1922. Serial N0. 602,021-

To all whom it may concern.' v

Be it known that I, FREDERICK KRANZ,

a citizen of the United States, and a resident of Geneva, in the county of Kane and State of Illinois, have invented certain new and useful Improvements in a Method of and Apparatus for Testing the Hearing; and I do hereby declare that the following is a full, clear,land exact description of the same, reference being had to the accom anying drawings, and to the numerals o reference marked thereon, which form a part ol this specification.

In testing the hearing, difliculty has been experienced heretofore in obtaining a reproducible standard of sound, so that the subject whose hearing has been tested b one apparatus might give a decidedly di i'erent result when tested by another appara tus, or even when tested by the same vapparatus at another time. Another cause, of inaccuracy in such tests heretofore has been that there is no'certain and dependable relation between the various adjustments of the apparatus and the loudness of the sound produced. Also there has been difiiculty in ascertaining the relation between the adjustments and the loudness at different pitches.

It is an object of the present invention to provide an apparatus which shall yield a sound whose loudness can be accurately detcrmined from the adjustments of the apparatus.

It is a further objectof this invention to provide such an apparatus which can be accurately reproduced.

It is a further object of this invention to produ'ce a sound-producin apparatus in which the relation between t e sound energy and the adjustments of the apparatus may be certainly known for sounds of various pitches It'is av further object of this invention to provide-fan apparatus for translating alternating electric current `into sound, so that the energy of the sound may be reliably computed from measurements of thel current used.

It is a further object of this invention to provide an apparatus which may be used for quickly and readil determining the pitch limits of audition or sounds of various intensities.

It is a further object of this invention to provide apparatus which c an quickly and certainly detect any irregularitieslin the relation between the pitch andthesensitiveness of the subject to sound.

Itis a further object of this invention to provide a method by which the presence of such irregularities may be :'certainl and uickly found with a minimum of la or on t e part of the investigator and a minimum of fatigue o r other inconvenience on the part of the subject.

Other and further. important objects of the invention will be apparent from the disclosures in the accompanying drawings and the following specification.

The invention (in a preferred form) is illustrated on the drawings and hereinafter more fully described.

On the drawings:

Figure 1 is a diagram of a` circuit showingone form of my invention.

Figures 2 and 3 are schematic sectional drawings illustratin the sound instrument used in t is circuit.

Figure 4 is a diagram of a portion of a circuit showing a modification.

As shown on the drawings: l

The circuit includes a source of alternating current. I prefer to use for this purpose a generator set such as is frequently used rin connection with radio work. It comprises an audion 10 connected in a way which is well understood by workers in the art, with an adjustable condenser 11 and an adjustable inductance 12. The output of this generator is connected'to an amplifier which includes an audion 13. The connections may include a key or other interrupter 14 for the purpose of controlling the. stopping and starting of the action.

'The output circuit of the amplier is in ductively connected as shown at 15 to a circuit which includes a succession of induc- 4tances 16 in series anda succession. of condensers 17 in shunt. Such arrangements, as

producing is well understood by workers in the eleetrical art, will, when properly chosen, read1-..

ly transmit a current of the fundamenta-l'.

frequency delivered by the amplifier but will shunt out harmonics or other higher frequency currents. Forthis reason, this portion of the circuit is spoken of as an 'electrical filter.

The output side of the filter is connected to a portion of the circuit including a succession of resistances 18 in series and 19 in shunt. As shown on the drawing, only one resistance 19 across the line is shown, but it is understood that any number that is desired may be used. Some or all of the resistances 18 may be adjustable and the resistances 19 likewise may be adjustable. If desired, the adjustment may extend to the complete elimination of some of the units consisting of a resistance 18 in each line and a resistance 19 across the lines. Also known resistances may be addedeither in series or in parallelv when desired. When the series resistances are changed, it is necessary to keep the resistance or' the two sides of the line of nearly equal resistance to ensure proper balancing. It is the function of this part of the circuit to regulate the intensity of the electrical oscillations arriving at the sound produ.,ing instrument. It does this by causing a predetermined diminution in the energy and is consequently called the attenu-atei'. The ,resistances therein are adjusted until the desired degree of attenuation is secured. A very convenient arrangement, but one which is not essential to the lfrom the electrical oscillations.

invention, is to have these resistances ar` ranged in sets, each set corresponding to a definite percentage of attenuation. The several sets can then be so connected that each is put into or removed from the circuit by the movement of a single switch handle.

From the inductive oonnectionl up to this point in the circuit, the arrangement is symmetrical, each inductance 16 being matched by an equal inductance 16 in the other line, and the capacities 17 being con nected between the inductances. In the same way, each resistance 18 is matched by al like resistance 18 in the other line and the shunts 19 are connected between the resistances 18.

At 2O there is introduced into one line an arrangement for measuring the current. Preferably a non-inductive arrangement is used, and a resistance 21 is placed in the other line to balance this non-inductive resistance. The current lmeasuring instrument chosen consists of a heating element shown at 20, which acts to heat the junctions of a thermo couple 22 which is connected to a delicate galvanometer or microammeter 23.

Either in place of the key 14 or in addition to it,. a switch 60 may be inserted just in front of the sound producing device. In order to keep the circuit symmetrical, a double pole switch is used.

At 2li there is shown connected across the line an instrument for producing sound The instrument which I have chosen for this purpose and which is known as a thermo-phone has been selected because it is possible to compute in dynes per square centimeter the pressure changes produced by this instrument when the energy of the electric current delivered to it is known. From values thus obtained, the sound energy may be calculated. The relations between the two depend on constants of the instrument which may be determined from its physical characteristics and so may be predicted with certainty. The thermo-phone consists of a strip of platinum 25 which is very thin. Preferably its thickness does not exceed .0002 centimeters. This strip is mounted in brass clamps 26 to which the wires 27 areconnected. rlhe strip 25 is enclosed ina chamber. This chamber may conveniently take the form of a telephone receiver. Current through the strip 25 heats it and so heats the air in the chamber. The/temperature and so the pressure of this air therefore changes with changes in the current. When the instrument is held to the ear, these pressure changes are' communicated to the 'eardrum through the open side of the chamber. The pressure within the chamber being a function of the temperature, the pressure changes may be accurately computed from the measurements of the current. As the heat produced is proportional ,to the square of Vthe current,l the heat changes will be larger and follow the current changes more faithfully if the alternating current issuperposedl on a direct current. -This direct current is supplied by a battery 31 and measured by an ammeter 32. The Valternating current from the transformer 15 is preventedy from reachingOthe'battery and ammeter by inductances 33, and the constant current fron the'battery 31 is prevented fronrreaching the inicroammeter 23 by capacities 34 inserted in'each line.

For working with very deaf subjects or for investigating sounds near the upper or lower pitch limit of audition, it is preferable to use a telephone receiver instead of the thermo-phone, because the telephone receiver gives a much louder sound. The constants of the telephone receiver, however, may not be predicted withcertainty and consequently4 each such receiver must be calibrated by comparison with a thermo-phone before it is used. When a telephone receiver is to be used, the circuit is modified as illustrated in Figure4 in which the inductances 16 and the capacityl 17 farthest from the transformer 15 are shown at the left of Figure 4. Adjustable resistances 38` in series in each side of the linefand adjustable resistances 39 across the line corresponding respectively to the resistance 18 and 19 are provided in this modification, but they are preferably placed upon the other side of the ineasuring instrument 20 and its balancing resistance 21, because the current used through a current which is vvery large compared' with the current iiowing through the telephone receiver, but the ratio between the two is easily computed from the resistances.

To facilitate the calculation of the amount,

by 'which the electrical current is attenuated 'by the resistances 38 and 39, it is important that the part of the circuit beyond the last small resistance 41. This small resistance.

41 is in series'wth larger resistances 40 of such magnitudes as tobring the wholey resov sistance of the circuit up to the value wished. Thus the impedance of the tele hone receiver has only a small effecten t e circuit as a whole, and this latterhas practically a constant resistance.

In an exemplification of this circuit which I havefound useful, each ofthe resista'nccs 40 was about 97 ohms, while the resistance 41 .was 6 ohms, which gave to the combination of resistances 40, 41 and telephone receiver 42 a substantially constant resistance at all frequencies of approximately 200 ohms. j y

'In the operation of the device, in the circuit shown in Figure 1, the generator and amplifier setup oscillatory electrical currents in the transformer 15. `The frequency of these oscillations is adjusted in large steps by'turning the dial of the adjustable capacity 11. The finer adjustments vof the frequency are effected by means of the adjustable inductance 12. The electrical filter made up of pro erly chosen inductances 16 andcapacities l? separates from the output of the transformer 15 all frequencies except the fundamental. Series resistances 18 and the shunt resistances' 19 cause a dissipation of part of the energy of the electrical oscillation or in other words this part of the circuit acts as an attenuator. These resistances are adjusted until the reading of the'microammeter 23 shows that the input into the thermo-phone is of the desired size. The current from the battery 31 is determined by the ammeter 32 and the temperature of the platinum stri From the reading o the microammeter 23 and the ammeter 32, therefore, the pressure changes produced by the thermo-phone may be determined.

The apparatus may be used by leaving the frequency adjustments at 11 and 12 constant, varying the adjustments at 18 and 19 for intensity changes until the limit of audibil- 25 is thus obtained.

ity for the particular frequency is found. The adjustmentsv at 11 and 12 may then be changed to elect a different frequency and the exploration by means of theA adjustments at 18 and 19 repeated.

While the apparatus described is available foi' this method, a better method is as 5 follows: The operator adjusts the capacity 11 so that the range of frequencies which he wishes to investigate may be produced by changes in the inductance 12. He then requests the subject who is listening to the sounds fr0-m the vthermophone 24 or telehone receiver 42r to vary the frequency back and forth over this frequency range by means of the handle controlling the indue-- tance 12. The operator adjusts the resistances 18 and 19 so as togive a definite sound energy output, and the subject, by means of his control of the inductance 12, determines over what portions of the frequency range, if any, the sound is audible. By noting the adjustment of the indicator on the inductance, the operator can' determine the lfrequencies of the audible portions of the frequency range and ,from the read 'ings of the microammeter and also ofthe resistances 18 and 19 in the. case of the use' of the telephone receiver, he can determine the yintensity of .the sound. The operator then adjusts the resistances 18 and 19 so as to change the sound intensity by a denite I, i

known amount and the subject again determines what portions of the frequency range are audible, vand readin@ are'lagain-taken to determine the frequencies and intensities of these portions, and so on. ,A preferred method of procedure is to have thefirst ad' justment. of the resistahces 18 and 19 such that the sound intensity is not suliicient. to

be audible to the subject at any point in the Y frequency range, and vsucceeding adjust-- ments ofthe resist-ences to be-suchthat'th'e intensity is increased in regular steps until al1 of the frequency range is audible, observations being made by the subject after each cha-nge of intensity. By noticing whether or not he can detect a stopping and starting of the, sound corresponding to the opening an-d closing of the key 60, the subject can make a-test las to whether a given intensity is beyond the limits of audibility. Sometimes a subject displays marked and abrupta changes` in sensibility to sounds withA changes of pitch, andthis method of testing is much more adapted to the discovery and exploration of these irregularities than a method dependent on changes of intensity using a series of constant pitches.

When'the subject is ayery deaf person or when the sounds being investigated are of such louT pitch or of such high pitch as to be very near the pitch limit of audibility, the arrangement shown in Figure 4 is used instead of that shown, in Figure 1. The procedure, however, is the same except that instead of the reading of the microammeter 23 giving directly the input into the sound producing instrument a fraction only of the current shown by the microammeter. 23 reaches the telephone receiver 42, this fraction being calcnlable from the values of the resistances 18 and 19. The telephone receiver itself, as already explained, .must be .calibrated by comparison with a thermophone.

I am aware that numerous details, both of the apparat-us and of the method, may be varied through a wide range without departing from the spirit ot' this invention, and I therefore do not purpose limiting the patent granted otherwise than is necessitated by the prior art.

I cla-im as my invention:

1. In an apparatus for investigating hearing, a. sound producing device capable of giving o'f sound energy bearing a predetermined ratio to the energy imparted to ,said device, means for imparting energy to said sound producing device, and means for measuring the energy thus imparted.

2. In a sound producing device, a generator of electrical alternating current, means for measuring the energy of said alternating current, and means for transforming the energy of the electrical alternating current into sound energy, the lat-ter f means having a known transformation ratio.

3. In a device of the class described, means for generating electrical alternating currents, means for adjusting the frequency of said alternating currents, means for amplifying said alternating currents, connections between the two including means for starting and stoppingA the operation, means` for separating from the amplified alternating currents all alternating currents of a frequency different from that for which adjust-ment is made, adjustable means for attenuat-ing the electrical alternating current,

FREDERICK W. KRANZ.

Witnesses CARLTON HILL, OSCAR HARTMANN. 

